Bicentric polyaxial locking screw and coupling element

ABSTRACT

A polyaxial orthopedic device for use with rod implant apparatus includes a screw having a curvate head, a bicentric locking shell disposed therearound, and a coupling element having a curvate tapered socket into which both the screw and the shell nest. The bicentric shell is slotted and has a semi-spherical interior volume into which the screw head is initially polyaxially held. The coupling element has an axial bore through which the screw and shell may be inserted, and a curvate bottom having a smaller radius of curvature than the shell, which provides a radially inward force on the locking shell when forced thereinto. This radially inward force causes the shell to crush lock against the head of the screw, therein locking the two at the instant angulation. In an initial disposition, however, the screw and the shell head remain polyaxially free with respect to each other, and to the coupling element. The coupling element also includes a transverse channel in which a rod may be disposed. In a first embodiment the channel is formed between a pair of upwardly extending members, and in a second embodiment, the channel is formed in the side of the element. In either case, a rod which is inserted in the channel seats against the top of the shell and the head of the screw. Engagement of a locking nut on the top of the coupling element provides a downward force against the rod, and forces the locking shell into the socket in the bottom of the bore, thereby locking the rod, locking shell, and screw simultaneously.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is a continuation-in-part of prior application U.S.Ser. No. 08/421,087, entitled "Anterior Spinal Polyaxial Locking ScrewPlate Assembly", filed Apr. 13, 1995, now U.S. Pat. No. 5,520,690.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a polyaxial screw and couplingapparatus for use with orthopedic fixation systems. More particularly,the present invention relates to a screw for insertion into spinal bone,and a coupling element polyaxially mounted thereto, via a slottedbicentric locking shell, for coupling the screw to an orthopedicimplantation structure, such as a rod, therein enhancing the efficacy ofthe implant assembly by providing freedom of angulation among the rod,screw and coupling element.

2. Description of the Prior Art

The bones and connective tissue of an adult human spinal column consistof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consist of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic andlumbar spine. For the purposes of this disclosure, however, the wordspine shall refer only to the cervical region.

Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of avertebral body, a pair of adjacent vertebral bodies, and a sequence ofvertebral bodies are shown, respectively. The spinal cord is housed inthe central canal 10, protected from the posterior side by a shell ofbone called the lamina 12. The lamina 12 includes a rearwardly anddownwardly extending portion called the spinous process 16, andlaterally extending structures which are referred to as the transverseprocesses 14. The anterior portion of the spine comprises a set ofgenerally cylindrically shaped bones which are stacked one on top of theother. These portions of the vertebrae are referred to as the vertebralbodies 20, and are each separated from the other by the intervertebraldiscs 22. The pedicles 24 comprise bone bridges which couple theanterior vertebral body 20 to the corresponding lamina 12.

The spinal column of bones is highly complex in that it includes overtwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complexities,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction. Genetic or developmentalirregularities, trauma, chronic stress, tumors, and disease, however,can result in spinal pathologies which either limit this range ofmotion, or which threaten the critical elements of the nervous systemhoused within the spinal column. A variety of systems have beendisclosed in the art which achieve this immobilization by implantingartificial assemblies in or on the spinal column. These assemblies maybe classified as anterior, posterior, or lateral implants. As theclassifications suggest, lateral and anterior assemblies are coupled tothe anterior portion of the spine, which is the sequence of vertebralbodies. Posterior implants generally comprise pairs of rods, which arealigned along the axis which the bones are to be disposed, and which arethen attached to the spinal column by either hooks which couple to thelamina or attach to the transverse processes, or by screws which areinserted through the pedicles.

"Rod assemblies" generally comprise a plurality of such screws which areimplanted through the posterior lateral surfaces of the laminae, throughthe pedicles, and into their respective vertebral bodies. The screws areprovided with upper portions which comprise coupling elements, forreceiving and securing an elongate rod therethrough. The rod extendsalong the axis of the spine, coupling to the plurality of screws viatheir coupling elements. The rigidity of the rod may be utilized toalign the spine in conformance with a more desired shape.

It has been identified, however, that a considerable difficulty isassociated with inserting screws along a misaligned curvature andsimultaneously exactly positioning the coupling elements such that therod receiving portions thereof are aligned so that the rod can be passedtherethrough without distorting the screws. Attempts at achieving properalignment with fixed screws is understood to require increased operatingtime, which is known to enhance many complications associated withsurgery. Often surgical efforts with such fixed axes devices cannot beachieved, thereby rendering such instrumentation attempts entirelyunsucessful.

The art contains a variety of attempts at providing instrumentationwhich permit a limited freedom with respect to angulation of the screwand the coupling element. These teachings, however, are generallycomplex, inadequately reliable, lack large angulation capacity, and lacklong-term durability. These considerable drawbacks associated with priorart systems also include difficulty properly positioned the rod andcoupling elements, and the tedious manipulation of the many small partsin the operative environment.

It is, therefore, the principal object of the present invention toprovide a pedicle screw and coupling element assembly which provides apolyaxial freedom of implantation angulation with respect to rodreception.

In addition, it is an object of the present invention to provide such anassembly which comprises a reduced number of elements, and whichcorrespondingly provides for expeditious implantation.

Accordingly it is also an object of the present invention to provide anassembly which is reliable, durable, and provides long term fixationsupport.

Other objects of the present invention not explicitly stated will be setforth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects of the invention are achieved by the presentinvention which is a polyaxial locking screw and coupling element foruse with rod stabilization and immobilization systems in the spine. Moreparticularly, the polyaxial screw and coupling element assembly of thepresent invention comprise a bone screw having a head which is curvatein shape, for example semi-spherical, a slotted bicentric locking shell,and a coupling element mounted thereto so as to be free to rotate priorto the secure fixation of the rod thereto, but which may be securelylocked at a given angulation once the rod is received. The couplingelement has an upper portion and a lower portion, the upper portionincluding a channel for receiving a rod therein, and the lower portionincluding a curvate socket for retaining and providing radial lockingforce against the slotted bicentric locking shell which in turn retainsthe head of the polyaxial screw. The coupling element further comprisesan exterior threading thereon for receiving a top locking nut.

The bicentric locking shell comprises a hollow semi-spherical body,having an interior volume which is ideally suited for holding thereinthe semi-spherical head of the screw. The top of the bicentric shell isopen so that a screwdriving tool, for example an allen wrench orthreaded post, may be used in conjunction with a corresponding recess inthe semi-spherical head of the screw to drive the screw into the desiredvertebral bone. The bicentric locking shell also includes a slot, suchthat the application of a radially compressive force may thus narrow theslot and decrease the interior volume. Conversely, the application of aradially outward force causes the slot to expand and the interior volumeto increase. In an unforced state, the head of the screw and the shellremain free to swing relative to one another, however, the applicationof a radially inward force causes the interior surface of the shell tocontract against the head of the screw, thereby crush locking the twoelements together.

The coupling element comprises a cylindrical body having a cylindricalbore extending axially therethrough. The diameter of the bore issufficient to permit the screw, its head, and the locking shell mountedabout the head to slide loosely from the top of the bore to the bottom.The bottom of the bore, however, has an inward curvate tapers which hasa radius of curvature which is less than that of the loose (non-radiallycompressed) locking shell, such that when the head of the screw and theshell are forced downward as far as it can be pushed, the shell isradially compressed so that the head of the screw can be locked to it.

The rod receiving upper portion of the coupling element comprises achannel wherein the rod of the implant apparatus is mounted. Moreparticularly, the walls of the hollow cylindrical body include opposingvertically oriented slots which extend downward from the top of theelement and which, therefore, which form a channel through the element.This channel divides the upper portion into two upwardly extendingmembers, between which the rod receiving channel is disposed. Thevertical slots have curvate bottom surfaces for receiving thereon therod.

In its initial, unlocked position (prior to being forced downward to thebottom of the axial bore of the element), the top of the bicentriclocking shell rests higher than the curvate bottoms of the slots, suchthat when the rod is placed in the channel, it seats against it and notthe curved bottom channel.

The upper portion of the upwardly extending members of the elementcomprises an external surface threading onto which a locking nut may bedisposed and translated downwardly. If a rod is disposed in the channel,the downward translation of the locking nut provides a correspondingdownward force onto the rod. The downward force on the rod translatesinto a downward force on the locking shell, causing it to be forceddownward into the bottom of the bore (or conversely, the translation ofthe nut on the external threading of the element draws the elementupward while holding the locking shell in place). This relative motionof the bicentric shell into the curvate bottom of the bore causes therod to seat against the curved bottom of the channel, the screw to beangularly locked in the locking shell, and the shell to be locked withinthe bottom of the element.

In a preferred variation, the locking nut comprises a cap nut which hasa central post which is designed to provide additional structuralsupport to the inner walls of the element at the top thereof, as well asproviding a central seating pressure point for locking the rod in thechannel. In either variation, the locking nut seats against the rod andprevents it from moving translationally, axially and rotationally.

The first step in the process of implanting this invention is to insertthe head of the screw into the bicentric locking shell. This may be doneprior to the surgery, such as by a surgical assistant, or even at themanufacturing site. The screw, and the shell, are then inserted throughthe axial bore of the coupling element, from the top, until the curvatetaper of the bottom of the bore cups and the shell. At this point, thescrew remains rotationally free to angulate relative to the lockingshell, and therefore, the coupling element. The bicentric shell alsoremains free to rotate within the bottom of the coupling element thusincreasing the total available rotation of the screw.

Once the screw has been so positioned, the screw, the shell, and thecoupling element are first aligned with respect to one another so thatthe appropriate screwdriving tool may be inserted down the axial bore,into the recess in the head of the screw, and used to rotate the bonescrew into the bone.

Subsequent to the insertion of the screw, the screwdriving tool may beremoved from the assembly (or, in the case of a threaded post, be leftin for alignment purposes), and the coupling element is then rotated tochange angular alignment relative to the screw and the shell. It is thecombined angulation freedom of the screw within the shell, and the shellwithin the bottom socket of the coupling element which defines the rangeof angles through which the coupling element may be angulated.

The rod of the implantation apparatus is then provided into the rodreceiving channel, and is positioned so that it seats against the top ofthe shell, which is slightly above the curvate bottom of the channel.The top locking nut is then introduced onto the threaded top portion ofthe upwardly extending members until the bottom of the nut (and a post,in the cap variation which includes a central post) seats against thetop of the rod. Continued tightening of the top locking nut causes thecoupling element to be drawn upward and/or the bicentric shell to bedriven downward, so that the screw head is crush locked within the shelland the shell within the coupling element.

Multiple screw and coupling element assemblies are generally necessaryto complete the full array of anchoring sites for the rod immobilizationsystem, however, the screw and coupling element assembly of the presentinvention is designed to be compatible with alternative rod systems sothat, where necessary, the present invention may be employed to rectifythe failures of other systems the implantation of which may have alreadybegun. For example, it is contemplated that this invention mayalternatively be embodied with a side channel, instead of a top channel,wherein the rod is received by the coupling element in its side. Such avariation may further require the use of a rod securing sleeve inconjunction with the top locking nut. Such a sleeve may comprise ahollow cylindrical body, having a curvate bottom surface for engaging arod, and which fits over the top of the element to seat against the rod.The top locking nut of a side loading variation would engage the sleeve,and the sleeve would in turn engage the rod.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of a human vertebra, which is representative of thetype for which the present invention is useful for coupling thereto arod apparatus.

FIG. 2 is a side view of a pair of adjacent vertebrae of the type shownin FIG. 1.

FIG. 3 is a posterior view of a sequence of vertebrae of the type shownin FIGS. 1 and 2.

FIG. 4 is a side view of a screw having a curvate head which is anaspect of the present invention.

FIG. 5 is a side view of the bicentric locking shell of the presentinvention, wherein critical interior features of the element are shownin phantom.

FIG. 6 is a side view of a first coupling element of the presentinvention wherein interior features of the element are shown in phantom.

FIGS. 7a and 7b are a side cross-sectional views of top locking nutswhich are aspects of the present invention; wherein FIG. 7a shows asimple open nut, and wherein FIG. 7b shows a cap nut having a centralpost.

FIG. 8 is a side cross-sectional view of the first embodiment of thepresent invention in its fully assembled disposition having a rodsecurely locked therein.

FIG. 9 is a side view of a second coupling element of the presentinvention having a side loading channel and wherein interior features ofthe element are shown in phantom.

FIG. 10 is a side cross-sectional view of a rod securing sleeve which isutilized with the side loading embodiment of the present invention.

FIG. 11 is a side cross-sectional view of the side loading embodiment ofthe present invention in its fully assembled disposition having a rodsecurely locked therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope.

Referring now to FIG. 4, a side view of the screw portion of the presentinvention, comprising a curvate head, is shown. The screw 120 comprisesa head portion 122, a neck 124, and a shaft 126. In FIG. 4, the shaft126 is shown as having a tapered shape with a high pitch thread 128. Itshall be understood that a variety of shaft designs are interchangeablewith the present design. The specific choice of shaft features, such asthread pitch, shaft diameter to thread diameter ratio, and overall shaftshape, should be made be the physician with respect to the conditions ofthe individual patient's bone, however, this invention is compatiblewith a wide variety of shaft designs.

The head portion 122 of the screw 120 comprises a semi-spherical shape,which has a recess 130 in it. It is understood that the semi-sphericalshape is a section of a sphere, in the embodiment shown the section isgreater in extent than a hemisphere, and it correspondingly exhibits anexternal contour which is equidistant from a center point of the head.In a preferred embodiment, the major cross-section of the semi-sphericalhead 122 (as shown in the two dimensional illustration of FIG. 4)includes at least 270 degrees of a circle.

The recess 130 defines a receiving locus for the application of a torquefor driving the screw 120 into the bone. The specific shape of therecess 122 may be chosen to cooperate with any suitable screw-drivingtool. For example, the recess 130 may comprise a slot for a screwdriver,a hexagonally shaped hole for receiving an allen wrench, or mostpreferably, a threading for a correspondingly threaded post. It isfurther preferable that the recess 130 be co-axial with the generalelongate axis of the screw 120, and most particularly with respect tothe shaft 126. Having the axes of the recess 130 and the shaft 126co-linear facilitates step of inserting the screw 120 into the bone.

The semi-spherical head portion 122 is connected to the shaft 126 at aneck portion 124. While it is preferable that the diameter of the shaft126 be less than the diameter of the semi-spherical head 122, it is alsopreferable that the neck 124 of the screw 120 be narrower than thewidest portion of the shaft 126. This preferable dimension permits thescrew to swing through a variety of angles while still being securelyjoined to the locking collar (as set forth more fully with respect toFIGS. 5, 8-9, and 11).

Referring now to FIG. 5, the bicentric locking shell of the presentinvention is shown in a side view, wherein phantom lines show theinterior structure of the elements along a diametrical cross section.The locking shell 150 comprises a slotted and tapered semi-sphericalbody 152 having a semi-spherical interior surface 154. The top surface156 of the shell 150 has an opening 155, through which the screwdrivingtool which is used to insert the screw 120 into the bone may access androtate the screw 120 through the shell 150.

The interior semi-spherical volume 158 is ideally suited for holding thehead portion 122 of the screw 120, and permitting the screw to rotatethrough a range of angles. The bottom 160 of the shell 150 has acircular hole 162, defined by annular lip 164, which forms the bottomentrance into the interior semi-spherical volume 158. It is understoodthat the head 122 of the screw 120 is held within the interiorsemi-spherical volume 158 by the relative size of the head 122 ascompared with the openings 155 and 162. More specifically, the annularlip 164 defines the circular opening 162 which has a diameter less thanthe diameter of the semi-spherical head 122 of the screw 120.

In order that the head 122 of the screw 120 may be inserted into thebicentric shell 150, the shell is slotted, perferably with a pluralityof slots 166, extending from the bottom opening 162 to points beyond themaximum diameter of the shell. This permits the shell 150 to be openedor closed, narrowed or expanded, in accordance with the application ofradial forces directed thereto. It is preferable that the relativecurvatures of the inner surface 154 of the shell and that of the head122 of the screw 120 are equal only when the shell has been contracted.The slot 166 permits the head portion 122 to be inserted into theinterior volume 158 of the shell 150, so that while being rotationallyfree to move once disposed therein, the head 122 may not be easilyremoved.

Referring now to FIG. 6, the coupling element 200 of the presentinvention is shown in a side view, wherein critical features of theinterior of the element are shown in phantom. The coupling element 200,which comprises a generally cylindrical tubular body having an axialbore 201 extending therethrough, may be conceptually separated into alower portion 202, and an upper portion 204, each of which shall bedescribed more fully hereinbelow.

First, with respect to the lower portion 202, the bottom interiorsurface 203 of the axial bore 201 has an inward curvate taper 209. Thediameter of the remainder of the axial bore 201 is such that the screw120 and the shell 150 may be inserted therethrough without applying aradially inward force against the shell 150. The inwardly curvate taper209 at the bottom of the axial bore 201, however, defines a socket 205,into which the shell 150 and the head 122 of the screw 120 disposedtherein may nest. Prior to its being fully driven into the socket 205,the screw 120 may be angulated relative to the shell 150, and the shellmay also be rotated and angulated relative to the coupling element 200.Once driven fully into the socket 205, however, the taper of theinterior surface 203 of the axial bore 201 provides the necessaryinwardly directed radial force to cause the shell 150 to crush lock tothe head 122 of the screw 120.

The upper portion 204 of the coupling element 200 includes a pair ofopposing, vertically oriented, slots 206 (one of which is shown in FIG.6) having rounded bottom surfaces 207. These slots 206 form a rodreceiving channel descending downward from the top 208 of the couplingelement 200. The channel, in turn, divides the wall cylindrical body ofthe upper portion 204 into a pair of upwardly extending members214a,214b. As shown in the embodiment illustrated in FIG. 6, thevertical distance from the top 208 of the channel to the curvate bottom207 thereof, is larger than the diameter of the rod which is to beprovided therein. This distance is necessarily larger than the diameterof the rod (see FIG. 8) so that the rod may be fully nested in thechannel. In addition, the depth of the bottom curvate surface 207 of thechannel is such that the top of the shell 150 is thereabove, prior toits full nesting into the socket 205.

The top 208 of the upper portion 204, which comprises upwardly extendingmembers 214a,214b, have disposed thereon a threading 216. This threading216 thereon, is ideally suited for receiving a top locking nut (seeFIGS. 7a and 7b).

Referring now to FIGS. 7a and 7b, a pair of alternative top locking nutsare shown in side cross-section view. First with respect to FIG. 7a, thenut 185 comprises an inner threading 186 which is intended to mate withthe threading 216 on the upwardly extending members 214a,214b of theupper portion 204 of the coupling element 200. The bottom surface 188 ofthe nut 185 is intended to seat against the top surface of the rod 250,but is permitted to rotate relative thereto, therein providing a meansfor driving the rod downward (as more fully described hereinbelow withrespect to the full assembly of the device, and with respect to FIG. 8).

With respect to the variation of the top locking nut of FIG. 7b, thelocking nut may further comprise a cap nut having a central post 190. Insuch a design, the nut includes a circular top portion 192, having anannular flange portion 194 extending downwardly therefrom at theperiphery thereof. The interior surface 196 of this flange portion 194comprises a threading 195 which is designed to engage the threading 216of the upwardly extending members 214a,214b. A cylindrical post 198 ispositioned at the center of the undersurface of the circular portion192, such that the distance between the exterior surface 199 of thecentral post 198 and the inner threaded surface 196 of the flangeportion 194 is sufficient for the upwardly extending members 214a,214b,on which the threadings 216 are disposed, to be engaged by the cap nut.Once fully engaged, the central post 198 provides enhanced strength tothe upwardly extending members 214a, 214b, so that they will not bendinwardly, toward one another, thus weakening the top locking nut'sholding ability. In addition, if the central post is sufficiently long,the bottom 197 thereof may be used to seat against the rod 250 (see FIG.8) to enhance the locking thereof to the coupling element 200.

Referring now to FIG. 8, which show side views of the initialdisposition of the screw, bicentric shell, and coupling element, and thefully locked coupling element, rod, shell and screw system,respectively, the preferred method of implantation and assembly isdescribed hereinbelow. First, a pre-drilled hole is provided in thebone, into which it is desired that the screw 120 be disposed. The holemay be pre-tapped, or the external threading 128 of the screw 120 mayinclude a self-tapping lead edge. In either event, the head 122 of thescrew 120 is inserted into the locking shell 150, such that it remainspolyaxially free to rotate and angulate relative thereto. The shell 150and the screw 120 are then translated downward, through the axial bore201 of the coupling element 200, until the shell 150 seats loosely inthe socket 205 at the curved tapered bottom of the axial bore 201. Atthis point in the assembly process, the screw 120 and the shell 150 havethe capacity to rotate relative to one another and within the couplingelement 200. In the initial position, the top surface 156 of the shell150 is disposed above the curvate bottom of the channel of the upperportion 204 of the coupling element 200, such that the notches 151 ofthe shell 150 are generally aligned with the channel.

By orienting the shell 150 and the screw 120 coaxially, a screwdrivingtool may engage the recess 130 in the head 122 of the screw 120 so thatit may be driven into the preformed hole in the bone.

Subsequent to the screw 120 being driven into the hole, the couplingelement 200 and the bicentric shell 150 may be rotated and agulatedrelative to the screw 120, to an angle such that support rod 250 may beeasily nested within the channel. In this initial disposition, however,the bottom 251 of the rod 250 seats on the top surface 156 of the shell150 and not fully on the bottom curved surface 207 of the channel.

After the rod 250 has been appropriately positioned, the top locking nut185 is threaded onto the threading 216 of the upwardly extending members214a,214b. The lower surface 188 of the nut 185 seats against the topsurface 252 of the rod 250. As the nut 185 rotates, and descendsrelative to the coupling element 200, the rod 250 is driven downward,causing the rod 250 and the shell 150 to translate downward slightly.This downward translation causes the tapered side walls 209 of thesocket 205 to compress against the locking collar 150, thereby causingthe slot 166 to narrow. This radial inward compression causes the head122 of the screw 120 to be crush locked to the inwardly curved surface154 of the shell 150.

In addition, the rod is locked between the bottom surface 188 of the nut185 and the top surface 156 of the shell 150. This locking prevents therod 250 from sliding relative to the assembled structure (along an axiswhich is perpendicular to the plane of FIG. 8). The full insertion ofthe top locking nut 185, therefore, locks the rod 250 in the channel ofthe coupling element 200, as well as the screw 120 within the shell 150and the shell to the socket 205.

Referring now to FIGS. 9, 10 and 11, an alternative, side loadingvariation of the invention is provided. Referring specifically to FIG.9, a coupling element 300 having a channel formed in the side thereof isillustrated in a side view, wherein critical features thereof are shownin phantom. This coupling element comprises an axial bore 301 which issimilar to the one in the first embodiment, through which the shell 150and the screw 120 may be inserted. The bottom of the axial bore 301 istapered so that the bicentric locking shell may initially seat looselyand polyaxially within the socket 305, and be securely locked thereinonly by the application of a downward force.

The coupling element comprises a lower portion 302, which is similar tothe lower portion of the first coupling element 200, as set forth abovewith respect to FIGS. 6 and 8. The remainder of the coupling element 300is divided into an intermediate portion 304 and an upper portion 306.The intermediate portion includes the side channel into which the rod250 is to be secured. In its initial disposition in the socket 305 ofthe lower portion 302, the top surface 156 of the shell 150 extendsupwards beyond the lower ledge 307 of the channel. The height of thechannel is equal to, or greater than the width of the rod and the extentto which the top surface 156 of the shell 150 rises above the lowerledge 307. This permits the rod 250 to be inserted into the channel andsubsequently for it to translate downward therein to force the shell 150downward in the socket 305 (thereby locking it securely therein).

The upper portion 306 of the coupling element 300 comprises a tubularsection which includes the top of the axial bore 301, and an exteriorthreading 316 which is suitable for engagement of either of the toplocking nuts set forth with respect to FIGS. 7a and 7b.

Referring to FIG. 10, the rod securing sleeve 330 which is necessary forreliably holding the rod 250 in the channel is shown in a side view. Therod securing sleeve comprises a hollow cylindrical section, having aninterior diameter which permits it to be dropped over the upper portion306 of the coupling element 300, and a lower annular surface 332 whichis shaped to cup the rod 250, and prevent its movement. The upperannular surface 334 of the sleeve 330 is suitably flat, so that thelower surface of the top locking nut may seat thereagainst and providethe downward force necessary to lock the entire system together.

Referring now to FIG. 11, in which the fully assembled second embodimentis shown in a side view, the assembly of this embodiment is described.As above, a pre-drilled hole is initially provided in the bone, intowhich it is desired that the screw 120 be disposed. The hole may bepre-tapped, or the external threading 128 of the screw 120 may include aself-tapping lead edge. In either event, the head 122 of the screw 120is inserted into the bicentric shell 150, such that it remainspolyaxially free to rotate and angulate relative thereto. The shell 150and the screw 120 are then translated downward, through the axial bore301 of the coupling element 300, until the shell 150 seats loosely inthe socket 305 at the curved tapered bottom of the axial bore 301. Atthis point in the assembly process, the screw 120 and the shell 150 havethe capacity to rotate relative to one another, and the shell 150 andcoupling element 300 have the capacity to polyaxially angulate as well.

By orienting the shell 150 and the screw 120 coaxially, a screwdrivingtool may engage the recess 130 in the head 122 of the screw 120 so thatit may be driven into the preformed hole in the bone.

Subsequent to the screw 120 being driven into the hole, however, thecoupling element 300 and the shell 150 and the screw 120 may be rotatedand angulated relative to one another, so that the screw is so angledthat support rod 250 may be properly nested within the side channel. Inthis initial disposition, however, the bottom 251 of the rod 250 seatson the top surface 156 of the shell 150 and not fully on the lower ledge307 of the channel.

After the rod 250 has been appropriately positioned, a rod securingsleeve 330 is placed over the upper portion 306 of the coupling element300 and dropped downward so that the bottom annular surface thereof maycup the top of the rod 250. Subsequently, the top locking nut 185 (or190) is threaded onto the threading 316 until the lower surface 188thereof seats against the upper annular surface 334 of the sleeve 330.As the nut 185 rotates and descends relative to the coupling element300, the rod securing sleeve 330 applies a downward force onto the rod250, which is, in turn, driven downward, causing the rod 250 and theshell 150 to translate downward slightly. This downward translationcauses the tapered side walls of the socket 305 to compress against theshell 150, thereby causing its slots 166 to narrow. This radial inwardcompression causes the head 122 of the screw 120 to be crush locked tothe inwardly curved surface 154 of the shell 150.

In addition, the rod is securely locked between the bottom surface 334of the rod securing sleeve 330 and the top surface 156 of the shell 150.This locking prevents the rod 250 from sliding relative to the assembledstructure (along an axis which is perpendicular to the plane of FIG.11). The full insertion of the top locking nut 185, therefore, locks therod 250 in the channel of the coupling element 300, as well as the screw120 within the shell 150.

While there has been described and illustrated embodiments of apolyaxial screw and coupling element assembly for use with posteriorspinal rod implantation apparatus, it will be apparent to those skilledin the art that variations and modifications are possible withoutdeviating from the broad spirit and principle of the present invention.The present invention shall, therefore, be limited solely by the scopeof the claims appended hereto.

We claim:
 1. A polyaxial screw and coupling element assembly for usewith orthopedic rod implantation apparatus, comprising:a polyaxial screwhaving a semi-spherical head; a bicentric locking shell, having at leastone axial slot such that said locking collar may be expanded orcontracted by the application of radial forces thereon, a curvateexterior surface, and a semi-spherical interior surface defining asemi-spherical volume in which said semi-spherical head may be initiallypolyaxially disposed so as to rotate and angulate relative theretothrough a range of angles including non-perpendicular angles; a couplingelement including,an axial bore through which said screw may beinserted, said bore having a narrowing curvate tapered bottom portion,said bottom portion forming a socket into which said bicentric lockingshell may be initially nested so that both the screw and the shell mayremain polyaxially disposed relative to each other and to the couplingelement, and said shell further being compression lockable in saidsocket by the application of a downward force thereon such that thetaper of the bore applies a radially inward compression force to closethe at least one slot and crush lock said semi-spherical interiorsurface of the shell onto the semi-spherical head of the screwindependent of the relative angulation of the screw or the shell to thecoupling element, a rod receiving channel, oriented transverse to saidaxial bore, in which the rod may be disposed and seated on a top surfaceof the shell, and an upper portion having an exterior threading thereon;and a top locking nut, mateable with said threading, for locking a rodin said channel and for applying therethrough the downward forcenecessary to compression lock the shell in the bottom socket portion ofthe axial bore whereby the screw head is crush locked to the interiorsurface of the shell.
 2. The assembly as set forth in claim 1, whereinsaid channel comprises a recess in the side of the coupling element. 3.The assembly as set forth in claim 1, wherein said upper portioncomprises a pair of upwardly extending members defining therebetweensaid channel.
 4. A orthopedic rod implantation apparatus havingpolyaxial screw and coupling elements, comprising:at least one elongaterod; a plurality of rod coupling assemblies, wherein at least one ofsaid assemblies comprises,a polyaxial screw having a semi-sphericalhead; a bicentric locking shell, having at least one axial slot suchthat said shell may be expanded or contracted by the application ofradial forces thereon, a curvate exterior surface, and a semi-sphericalinterior surface defining a semi-spherical volume in which saidsemi-spherical head may be initially polyaxially disposed so as torotate and angulate relative thereto through a range of angles includingnon-perpendicular angles; a coupling element including,an axial borethrough which said screw may be inserted, said bore having a narrowingcurved tapered bottom portion, said bottom portion forming a socket intowhich said shell may be initially nested so that the screw and shell mayremain polyaxially disposed relative to one another and to the couplingelement, and said shell further being compression lockable in saidsocket by the application of a downward force thereon such that thetaper of the bore applies a radially inward compression force to closethe at least one slot and crush lock said semi-spherical interiorsurface of the shell to the semi-spherical head of the screw independentof the relative angulation of the screw or the shell to the couplingelement, a rod receiving channel, oriented transverse to said axialbore, in which the rod may be disposed and seated on a top surface ofthe shell, and an upper portion having an exterior threading thereon;and a top locking nut, mateable with said threading, for locking a rodin said channel and for applying therethrough the downward forcenecessary to compression lock the shell in the bottom socket portion ofthe axial bore whereby the screw head is crush locked to the interiorsurface of the shell.
 5. The assembly as set forth in claim 4 whereinsaid channel comprises a recess in the side of the coupling element. 6.The assembly as set forth in claim 4, wherein said upper portioncomprises a pair of upwardly extending members defining therebetweensaid channel.